The experiments described in this research proposal are designed to quantitatively evaluate the rate of bicarbonate reabsorption (JtCO2) and the capacity of the rat superficial proximal convoluted tubule to reabsorb bicarbonate during normal systemic acid-base states and during states of altered acid-base balance. In all studies, the technique of in vivo microperfusion will be used to exactly control the intraluminal bicarbonate concentration and delivery rate. The concentration of total CO2 in the collected perfusate will be measured using microcalorimetry. During normal acid-base status, tubules will be perfused with a wide range of perfusate bicarbonate concentrations to determine effects of intraluminal concentration on JtCO2, and at perfusion rates of 20 to 50 nl per minute to determine the effects of load on JtCO2. Using similar protocols, the effects of extracellular fluid volume expansion, acute and chronic metabolic acidosis, acutre metabolic alkalosis and acute respitory acidosis and alkalosis on the capacity of the proximal tubule to reabsorb bicarbonate will be evaluated. The proposed experimnents will make it possible to determine whether proximal tubule JtCO2 is a function of bicarbonate dilivery alone without influence of systemic acid-base conditions or whether systemic acid-base status can influence JtCO2 through mechanisms independent of the load presented to the tubule.